1. Field of the Invention
The present invention relates to fuel injectors and nozzles, and more particularly, to check valves used in fuel injectors and nozzles for gas turbine engines.
2. Description of Related Art
A variety of devices and methods are known in the art for regulating fuel flow in fuel injectors and nozzles during engine start up and low flow conditions. Of such devices, many are directed to check valves for preventing or reducing fuel flow through injectors and nozzles prior to start up.
During the starting cycle of a gas-turbine engine, fuel is supplied at a set mass flow rate to a manifold by the fuel pump, which creates the pressure needed to deliver the required mass flow rate. The manifold supplies fuel to a plurality of fuel injectors. Many fuel injectors contain a check valve to help ensure that fuel is not introduced to the combustor until a set manifold pressure is reached. Due to manufacturing tolerances and spring force variations, the point at which the check valves open, or “crack,” can vary significantly.
If the fuel injector is sized for a low pressure, high flow system, the effect of variations in crack pressure can cause serious maldistribution of fuel flow from one injector to the next. The manifold pressure can be just high enough to open a few of the check valves, allowing the fuel to pass virtually unrestricted out of the open injectors due to the injector passage sizes, while other injector check valves remain closed and therefore not flowing. Particularly during the ignition sequence, when manifold pressures are low, some injectors can be in a no flow state, while other injectors are flowing at relatively high rates. This uneven distribution of fuel can create difficulties in achieving ignition and light around. Even at typical ground idle fuel flow rates, it is possible for some of the injectors to be in the no flow condition. This can lead to poor low power emissions due to uneven distribution of fuel within the combustor. Moreover, the fact that some injectors are not flowing can cause combustor stress due to temperature gradients, i.e. hot spots and cold spots, around the combustor.
A typical check valve includes a piston biased to seal against a seat through some mechanical means, such as a spring. The crack pressure is determined by the piston area and the spring preloading force. Once the fuel pressure is high enough to overcome the spring preload, the piston lifts from its seat to allow flow. If the pressure drops below the spring preloading force, the valve will close and reseal. Due to variations in the spring preloading and piston area from check valve to check valve, injectors employing such check valves can have widely varied crack pressures within a single combustor, leading to the problems described above.
One solution to this problem is to use scheduling valves instead of check valves. A typical scheduling valve employs a match grind between a shaft and a bore of a cylinder. The precision is high enough to provide high precision porting that will control the fuel flow proportional to manifold fuel pressure. As the pressure builds, the open area of the scheduling valve increases. There is often a check valve integral with the scheduling valve. Back pressure from the scheduling feature helps ensure that there is uniform flow throughout the manifold.
Scheduling valves are effective at overcoming the short comings described above for traditional check valves. However, scheduling valves are relatively expensive to manufacture due to the high tolerances required. Scheduling valves are also prone to hysteresis. The hysteresis occurs because of stack up tolerances and material property differences. There is also a difference in the perceived piston area, which causes the pressure load to be different depending on where the piston is in relation to its stroke. The precision porting is also prone to malfunction due to particles and contaminants in the fuel, which can drastically affect the flow.
U.S. Pat. No. 5,918,628 to Harding describes a multi-stage check valve in which a valve poppet moves within a valve housing as fuel pressure on the valve poppet increases. Radial through-holes of two different diameters are staged axially along the valve poppet. Just after the poppet unseats from the valve housing, fuel flows first only through a small through-hole providing a low flow rate. Then as fuel pressure increases and the poppet moves further into the valve housing, larger through-holes in the poppet are opened to allow for a larger fuel flow through the check valve. This configuration provides fuel staging with relatively low pressure drop. However, the valve described in the Harding Patent requires the piston to be matched closely to the sleeve portion to ensure schedule “rigidity” or relative separation of the primary and second state. Thus there can not be a large tolerance around the piston. One major restriction in the Harding valve is that the coupling of the opening of the ports relies upon one single spring. This can drive the design to have undesirable effects on the physical envelope required to contain the design, i.e., the staging requirements in the Harding design drive the size of the check valve because there is only one spring for both high and low pressure ports. The Harding valve also does not necessarily seal the second stage from the primary stage.
Such conventional methods and systems generally have been considered satisfactory for their intended purpose. However, there still remains a continued need in the art for a valving device that can provide improved operability across a range of fuel flow rates. There also remains a need in the art for such a valving device that is easy to make and use. The present invention provides a solution for these problems.